CSE 120: Homework #4

Fall 2024

Due: Wednesday December 4 at 11:59pm

Each question is worth 5 points, except for (3) which is worth 10 points.

  1. On a Unix-style file system, how many disk read operations are required to open and read the first block of the file "/usr/include/X11/Xlib.h"? Assume (1) that the superblock is in memory, but nothing else; (2) that all directories and inodes are one block in size; (3) that once a block is read, it is cached in memory (and does not need to be read again); (4) inode timestamps (like last access) do not need to be updated (the inode does not need to be written after access).

  2. Consider a UNIX-style inode with 10 direct pointers, one single- indirect pointer, and one double-indirect pointer only. Assume that the block size is 4KB (including indirect blocks), and that the size of a pointer is 4 bytes. How large a file can be indexed using such an inode?

  3. (10 pts) The original Berkeley Fast File System increased the Unix file system block size from 512 bytes to 4096 bytes. Concerned about internal fragmentation, the FFS also introduced the ability to end a file with a small fragment. A disk block could be broken up into small fixed-size fragments, each of which could be used to store the ends of different files. For instance, a file of size 5000 bytes would need two blocks (8192 bytes) to store on disk, resulting in 3192 bytes lost to internal fragmentation. With 1024-byte fragments, though, the file could be stored with one full-sized block (4096 bytes) and one 1024-byte fragment, requiring 5120 bytes on disk to store the file and reducing internal fragmentation to just 120 bytes. The tradeoff is that managing fragments increases the complexity of the file system implementation.

    1. My laptop has 220 (1024 * 1024) files on it. Assume the disk block size is 4KB and the average amount of internal fragmentation is 2KB per file. How much storage is wasted due to internal fragmentation in the file system on my laptop?
    2. Assume that, with fragments, the average amount of internal fragmentation goes down to 256 bytes per file. How much storage is wasted due to internal fragmentation when using fragments?
    3. Assume that you would receive the same benefits for your laptop. Would you want the file system to use fragments to save space? Why or why not?

  4. Consider a file archival system, like the programs zip or tar. Such systems copy files into a backup file and restore files from the backup. For example, from the zip documentation:
    The zip program puts one or more compressed files into a single zip archive, along with information about the files (name, path, date, time of last modification, protection, and checksum information to verify file integrity).

    When a file is restored, it is given the same name, time of last modification, protection, and so on. If desired, it can even be put into the same directory in which it was originally located.

    Can zip restore the file into the same inode as well? Briefly explain your answer.

  5. [Silberschatz] Consider a system that supports 5000 users. Suppose that you want to allow 4990 of these users to be able to access one file.
    1. How would you specify this protection scheme in Unix?
    2. Could you suggest another protection scheme that can be used more conveniently for this purpose than the scheme provided by Unix? (Hint: Perhaps by violating a principle...)

  6. [Silberschatz] How does a file cache help improve performance? Why do systems not use much larger caches if they are so useful?

  7. Consider a program that executes a loop that issues a read I/O to a storage device and waits I milliseconds for the I/O to complete, and then computes on the data returned for X milliseconds, and then repeats.

    For various values of I and X, compute the percentage of time that the program spends waiting for I/O and fill in the following table. If I and X are both 1 ms, for example, then the program spends 50% of its time waiting for I/O.

    X
    100 ms 10 ms 1 ms 0.1 ms
    25 ms (Data Center)  
    5 ms (HDD)
    I 0.1 ms (SSD)
    0.005 ms (PCM)
    0.001 ms (RAM)
voelker@cs.ucsd.edu